Myeloperoxidase from granular leucocytes of humans with a normal differential cell count will be isolated and characterized. Inhibition studies will be conducted in the event the enzyme is (a) devoid or peroxidatic-deaminase activity with respect to amino acid substrates (an activity which has been proved for leukemic granulocytes) or (b) exhibits enzymatic and kinetic parameters which differ strinkingly from the reported values and descriptions of leukemic myeloperoxidase. An inhibitor will be sought to specifically or preferentially inhibit leukemic myeloperoxidase (rather than the homologous enzyme from "normal" granulocytes).